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At least 433 records · Page 24Linked to original sources

PubMed1991

Epidemiological evidence for the 'two-disease hypothesis' in Hodgkin's disease.

This study relates to 1803 newly diagnosed cases of Hodgkin's Disease (HD) registered between 1984 and 1988 by a specialist leukaemia-lymphoma registry covering approximately half of England and Wales. In addition to registration data the analyses use routine census data. Rye classification is available for the majority of cases with only 7% being unclassified. The data confirm that the young adult peak in HD occurrence is attributable to the nodular sclerosing subtype (NS), and there is some evidence that NS has a different geographical distribution by county to the other Rye subtypes. Because of possible geographical biases in the classification, subsequent spatial analyses are disaggregated by age at diagnosis with particular emphasis being placed on the two age groups 0-34 and 50-79 years. For these, trends of risk by areal socioeconomic status are in opposite directions and the effects of urban-rural status while in the same direction also differ significantly. Allowance for these does not, however, explain the significant difference between the county distributions of the two groups. Patterns of spatial clustering are quite distinct with evidence for local spatial aggregation amongst younger cases.

Adolescent↗

PubMed2005

Adaptive smoothing via contextual and local discontinuities.

A novel adaptive smoothing approach is proposed for noise removal and feature preservation where two distinct measures are simultaneously adopted to detect discontinuities in an image. Inhomogeneity underlying an image is employed as a multiscale measure to detect contextual discontinuities for feature preservation and control of the smoothing speed, while local spatial gradient is used for detection of variable local discontinuities during smoothing. Unlike previous adaptive smoothing approaches, two discontinuity measures are combined in our algorithm for synergy in preserving nontrivial features, which leads to a constrained anisotropic diffusion process that inhomogeneity offers intrinsic constraints for selective smoothing. Thanks to the use of intrinsic constraints, our smoothing scheme is insensitive to termination times and the resultant images in a wide range of iterations are applicable to achieve nearly identical results for various early vision tasks. Our algorithm is formally analyzed and related to anisotropic diffusion. Comparative results indicate that our algorithm yields favorable smoothing results, and its application in extraction of hydrographic objects demonstrates its usefulness as a tool for early vision.

Algorithms↗

PubMed2002

Biofilms as complex differentiated communities.

Prokaryotic biofilms that predominate in a diverse range of ecosystems are often composed of highly structured multispecies communities. Within these communities metabolic activities are integrated, and developmental sequences, not unlike those of multicellular organisms, can be detected. These structural adaptations and interrelationships are made possible by the expression of sets of genes that result in phenotypes that differ profoundly from those of planktonically grown cells of the same species. Molecular and microscopic evidence suggest the existence of a succession of de facto biofilm phenotypes. We submit that complex cell-cell interactions within prokaryotic communities are an ancient characteristic, the development of which was facilitated by the localization of cells at surfaces. In addition to spatial localization, surfaces may have provided the protective niche in which attached cells could create a localized homeostatic environment. In a holistic sense both biofilm and planktonic phenotypes may be viewed as integrated components of prokaryote life.

Bacteria↗

PubMed2002

Surgery of low-grade gliomas near speech-eloquent regions: brainmapping versus preoperative functional imaging.

The identification of eloquent areas is of utmost importance in the surgery of tumors located near speech-eloquent brain areas, since the classical concept of a constant localization was proven to be untrue and the spatial localization of these areas may show large interindividual differences. Some neurosurgical centers apply intraoperative electrophysiological methods that, however, necessitate the performance of surgery in the awake patient. This might be a severe burden both for the patient and the operating team in a procedure that lasts several hours; in addition, electrical stimulation may generate epileptic seizures. Alternatively, methods of functional brain imaging (e.g., PET, fMRI, MEG) may be applied, which allow individual localization of speech-eloquent areas. Matching of these image data with a conventional 3D-CT or MRI now allows the exact transfer of this information into the surgical field by neuronavigation. Whereas standards concerning electrophysiological stimulation techniques that could prevent a permanent postoperative worsening of language are available, until now it remains unclear whether the resection of regions shown to be active in functional brain imaging will cause a permanent postoperative deficit.

Brain Mapping↗

PubMed2004

Contrast sensitivity deficits in inferred magnocellular and parvocellular pathways in retinitis pigmentosa.

PURPOSE: To define the contrast sensitivity deficits of patients with retinitis pigmentosa (RP) under testing conditions designed to emphasize threshold mediation by either the magnocellular (MC) or parvocellular (PC) pathway. METHOD: Contrast sensitivity was measured with spatially localized, narrow-band test patterns at peak spatial frequencies ranging from 0.25 to 8 cycles per degree (cpd), using a steady-pedestal paradigm (brief presentation of the test stimulus against a continuously presented luminance pedestal) and a pulsed-pedestal paradigm (simultaneous brief presentation of the test stimulus and luminance pedestal) to favor the MC and PC pathways, respectively. The contrast sensitivity functions of 12 patients with RP who had visual acuities ranging between 20/12.5 and 20/40 were compared to those of 10 visually normal, age-equivalent control observers. RESULTS: Five of the patients with RP who had Snellen visual acuities better than 20/25 had contrast sensitivity functions that were within the normal limits at all spatial frequencies for both testing paradigms. The other seven patients with RP had reduced contrast sensitivities for both paradigms, with the greatest reduction in sensitivity occurring at the highest spatial frequency. Their contrast sensitivity deficits were equivalent for the steady- and pulsed-pedestal paradigms. CONCLUSIONS: As observed in previous studies, the degree of contrast sensitivity loss shown by the patients with RP was greatest at the highest stimulus spatial frequency. However, in comparison to prior studies of contrast discrimination in patients with RP, there was no evidence of a preferential contrast sensitivity loss within the MC pathway. This apparent discrepancy is attributed to differences in the test targets and psychophysical judgments that were used in the studies, which emphasizes the importance of task characteristics in evaluating relative deficits within the MC and PC processing streams in visual disorders.

Adult↗

PubMed1991

Auditory localization under conditions of unilateral fitting of different hearing aid systems.

The spatial localization function of hearing-impaired listeners, usually fitted bilaterally with BTE, ITE or ITC devices, was tested under conditions of unilateral fitting of each of their own hearing aids, and unilateral fitting of stock versions of each of the other types. For the BTE wearers average localization accuracy and individual variability were not greatly changed when wearing only their left ear device, compared with bilateral aided performance. In ITE wearers, unilateral fitting in either ear led to somewhat poorer performance than bilateral. In the ITC wearers unilateral fitting produced inconsistent outcomes. Both BTE and ITE wearers fared poorly when fitted unilaterally with stock forms of devices 'foreign' to them, whereas ITC wearers did not show such a contrasting outcome. A group of non-impaired listeners showed severe disruption of localization under unilateral BTE and ITC hearing aid conditions, and to a lesser extent with ITEs. Results for the hearing-impaired listeners are interpreted in terms of adaptation to different usage patterns, with BTE wearers suggested as having adapted to their own systems unilaterally as well as bilaterally.

Auditory Threshold↗

PubMedDate not supplied

Turning on and off with excitation: the role of spike-timing asynchrony and synchrony in sustained neural activity.

Delay-related sustained activity in the prefrontal cortex of primates, a neurological analogue of working memory, has been proposed to arise from synaptic interactions in local cortical circuits. The implication is that memories are coded by spatially localized foci of sustained activity. We investigate the mechanisms by which sustained foci are initiated, maintained, and extinguished by excitation in networks of Hodgkin-Huxley neurons coupled with biophysical spatially structured synaptic connections. For networks with a balance between excitation and inhibition, a localized transient stimulus robustly initiates a localized focus of activity. The activity is then maintained by recurrent excitatory AMPA-like synapses. We find that to maintain the focus, the firing must be asynchronous. Consequently, inducing transient synchrony through an excitatory stimulus extinguishes the sustained activity. Such a monosynaptic excitatory turn-off mechanism is compatible with the working memory being wiped clean by an efferent copy of the motor command. The activity that codes working memories may be structured so that the motor command is both the read-out and a direct clearing signal. We show examples of data that is compatible with our theory.

Action Potentials↗

PubMed2000

Growth and decay of discrete nonlinear Schrodinger breathers interacting with internal modes or standing-wave phonons

We investigate the long-time evolution of weakly perturbed single-site breathers (localized stationary states) in the discrete nonlinear Schrodinger equation. The perturbations we consider correspond to time-periodic solutions of the linearized equations around the breather, and can be either (i) spatially localized or (ii) spatially extended. For case (i), which corresponds to the excitation of an internal mode of the breather, we find that the nonlinear interaction between the breather and its internal mode always leads to a slow growth of the breather amplitude and frequency. In case (ii), corresponding to interaction between the breather and a standing-wave phonon, the breather will grow provided that the wave vector of the phonon is such that the generation of radiating higher harmonics at the breather is possible. In other cases, breather decay is observed. This condition yields a limit value for the breather frequency above which no further growth is possible. We also discuss another mechanism for breather growth and destruction which becomes important when the amplitude of the perturbation is non-negligible, and which originates from the oscillatory instabilities of the nonlinear standing-wave phonons.

Journal Article↗

PubMed2006

Localized states in the generalized Swift-Hohenberg equation.

The Swift-Hohenberg equation with quadratic and cubic nonlinearities exhibits a remarkable wealth of stable spatially localized states. The presence of these states is related to a phenomenon called homoclinic snaking. Numerical computations are used to illustrate the changes in the localized solution as it grows in spatial extent and to determine the stability properties of the resulting states. The evolution of the localized states once they lose stability is illustrated using direct simulations in time.

Journal Article↗

PubMed1996

Emergence of simple-cell receptive field properties by learning a sparse code for natural images.

The receptive fields of simple cells in mammalian primary visual cortex can be characterized as being spatially localized, oriented and bandpass (selective to structure at different spatial scales), comparable to the basis functions of wavelet transforms. One approach to understanding such response properties of visual neurons has been to consider their relationship to the statistical structure of natural images in terms of efficient coding. Along these lines, a number of studies have attempted to train unsupervised learning algorithms on natural images in the hope of developing receptive fields with similar properties, but none has succeeded in producing a full set that spans the image space and contains all three of the above properties. Here we investigate the proposal that a coding strategy that maximizes sparseness is sufficient to account for these properties. We show that a learning algorithm that attempts to find sparse linear codes for natural scenes will develop a complete family of localized, oriented, bandpass receptive fields, similar to those found in the primary visual cortex. The resulting sparse image code provides a more efficient representation for later stages of processing because it possesses a higher degree of statistical independence among its outputs.

Algorithms↗

PubMed1987

[Localization of antigenic determinants on a molecule of trophoblast-specific beta 1-glycoprotein].

Spatial localization of antigenic determinants of trophoblast-specific beta I-glycoprotein (TSG) has been elucidated using chemical modifications of the sugar and protein moieties of the molecule. Various deglycosylation procedures of TSG afforded fragments slightly soluble even in the presence of powerful detergents. Treatment of TSG with boric acid and its salts, accompanied with a conformational change of the sugar moiety, failed to alter conformation of the protein portion as evidenced by CD spectral data. This modification was found to increase the antigenic activity of TSG only scarcely. Modification of tryptophane or tyrosine residues of TSG changed spatial structure of the protein portion to can be a considerable loss of the TSG antigenic activity. The data obtained led to the conclusion that antigenic determinants of TSG are localized at the protein portion of the molecule and are topographic. A tryptophane residue is an indispensable constituent of the antigenic determinants.

Circular Dichroism↗

PubMed1997

Interactive image-guided surgery of the spine--use of the ISG/Elekta Viewing Wand to aid intraoperative localization of a sacral osteoblastoma.

The intraoperative localization of small osteoid osteomas and osteoblastomas of the spine is often difficult. The authors report a patient with a small sacral osteoblastoma in whom the experimental use of an Interactive Image-Guidance Stereotactic system, to aid localization, is described. The Elekta Viewing Wand is a spatial localization device primarily designed for intracranial procedures. We evaluated its potential role in the localization and minimally invasive excision of a sacral osteoblastoma in this patient. A basic assessment of the limitations encountered in extracranial use of this system is presented and possible solutions to minimize these problems are discussed.

Adult↗

PubMed1989

[Analysis of "scalp potential fields" and the tridimensional location of the sources of epileptic activation in humans].

The method of multi-step dipole localization with potential charts preanalysis was applied to EEG spatial localization of discharge activity in 24 epileptic patients. The results were juxtaposed to the X-ray contrasting techniques and computerized tomography. Spherical symmetrical 8-layer head model was used to interprete the results. The data on three-dimensional localization of focal and generalized paroxysmal activities are presented. The role of this technique in differentiating the primarily generalized activity from secondary foci is outlined. The generation and recording sites epileptic activity could diverge largely due to the primary source orientation. For correct EEG interpretation, it is important that the primary generation zone be reconstructed. This is an additional field of application of this technique.

Action Potentials↗

PubMed2006

Spatial patterns of malaria in the Amazon: implications for surveillance and targeted interventions.

A measure of local spatial association, G(i)*(d), is applied to test for the presence of malaria clusters in a colonization area in the Brazilian Amazon. Clusters of high and low malaria rates at different moments in time are identified. They suggest unambiguous spatial patterns of transmission, most likely linked to the social and natural habitat. Results imply that a comprehensive identification of the determinants of malaria transmission requires a spatial framework of analysis, and that control strategies must be spatially targeted and guided by a surveillance system that constantly learns the specificities of local transmission and adapts interventions to them.

Brazil↗

PubMed1998

Spatial scales of desert locust gregarization.

Central to swarm formation in migratory locusts is a crowding-induced change from a "solitarious" to a "gregarious" phenotype. This change can occur within the lifetime of a single locust and accrues across generations. It represents an extreme example of phenotypic plasticity. We present computer simulations and a laboratory experiment that show how differences in resource distributions, conspicuous only at small spatial scales, can have significant effects on phase change at the population level; local spatial concentration of resource induces gregarization. Simulations also show that populations inhabiting a locally concentrated resource tend to change phase rapidly and synchronously in response to altered population densities. Our results show why information about the structure of resource at small spatial scales should become key components in monitoring and control strategies.

Journal Article↗

PubMed2005

Localized waves in nonlinear oscillator chains.

This paper reviews results about the existence of spatially localized waves in nonlinear chains of coupled oscillators, and provides new results for the Fermi-Pasta-Ulam (FPU) lattice. Localized solutions include solitary waves of permanent form and traveling breathers which appear time periodic in a system of reference moving at constant velocity. For FPU lattices we analyze the case when the breather period and the inverse velocity are commensurate. We employ a center manifold reduction method introduced by Iooss and Kirchgassner in the case of traveling waves, which reduces the problem locally to a finite dimensional reversible differential equation. The principal part of the reduced system is integrable and admits solutions homoclinic to quasi-periodic orbits if a hardening condition on the interaction potential is satisfied. These orbits correspond to approximate travelling breather solutions superposed on a quasi-periodic oscillatory tail. The problem of their persistence for the full system is still open in the general case. We solve this problem for an even potential if the breather period equals twice the inverse velocity, and prove in that case the existence of exact traveling breather solutions superposed on an exponentially small periodic tail.

Journal Article↗

PubMed1993

Localized 1H NMR measurement of glucose consumption in the human brain during visual stimulation.

Spatially localized 1H NMR spectroscopy has been applied to measure changes in brain glucose concentration during 8-Hz photic stimulation. NMR spectroscopic measurements were made in a 12-cm3 volume centered on the calcarine fissure and encompassing the primary visual cortex. The average maximum change in glucose levels was 0.34 mumol.g-1 (n = 5) at 15 min; glucose level had turned toward resting level at 25 min. The glucose change was used to calculate the increase of glucose cerebral metabolic rate in the visual cortex region for individual subjects by using the Michaelis-Menten model of glucose transport on the assumption of constant transport kinetics. The glucose cerebral metabolic rate was calculated to increase over the nonstimulated rate by 22% during the first 15 min of photic stimulation. A model in which the glucose metabolic rate gradually decreases during stimulation was proposed as a possible explanation for the recovery of brain glucose and previously measured lactate concentrations to prestimulus values after 15 min.

Blood Glucose↗

PubMed2000

Localized perturbations in binary fluid convection with and without throughflow.

Dynamics and structure of spatially localized convective perturbations in binary fluid layers heated from below and the effect of a plane horizontal Poiseuille throughflow on them are investigated. Fronts and pulse-like wave packets formed out of the three relevant perturbations-two oscillatory ones and a stationary one-are analyzed after evaluating the appropriate saddle points of the three respective dispersion relations of the linear field equations over the complex wave number plane. Front and pulse properties are elucidated in quantitative detail as a function of small throughflow Reynolds numbers for different Soret coupling strengths psi including the pure fluid limit psi=0 in comparison with the appropriate Ginzburg-Landau amplitude equation approximations. Furthermore, small amplitude pulses and fronts obtained from solving the full nonlinear field equations numerically are presented to check and compare with the linear results.

Journal Article↗